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| United States Patent |
5,548,113
|
|
Goldberg
, et al.
|
August 20, 1996
|
Co-axial detection and illumination with shear force dithering in a
near-field scanning optical microscope
Abstract
A near-field optical microscope is provided having a tip or tapered optical
fiber light guide mounted directly in a hole drilled in the center of a
microlens which itself is mounted in a piezoelectric tube for dithering.
The tip of the tapered fiber is positioned at the front focal point of the
lens. Light emanating from the region just about the tip is thus collected
and collimated by the lens. A second lens, further down the optical path
next focuses the beam into another fiber for transmission to a photon
detection and/or spectroscopic apparatus. This second fiber can
alternatively be replaced by one or more image processing lenses or a
coherent fiber bundle with the light being transmitted to a position
sensitive detector to provide a direct image of the tip region. The
microscope also includes a shear force dithering apparatus for control of
fiber tip to sample distance.
| Inventors:
|
Goldberg; Bennett B. (Newtonville, MA);
Ghaemi; Hadi F. (Brookline, MA)
|
| Assignee:
|
Trustees of Boston University (Boston, MA)
|
| Appl. No.:
|
216887 |
| Filed:
|
March 24, 1994 |
| Current U.S. Class: |
250/234; 250/216; 250/306; 359/368 |
| Intern'l Class: |
H01J 003/14; H01J 005/16 |
| Field of Search: |
250/307,306,226,227.26,216,234
359/368,385
|
References Cited
U.S. Patent Documents
| 4604320 | Aug., 1986 | Okamoto et al. | 428/423.
|
| 4659429 | Apr., 1987 | Isaacson et al. | 430/5.
|
| 4662747 | May., 1987 | Isaacson et al. | 359/368.
|
| 4725727 | Feb., 1988 | Harder et al. | 250/227.
|
| 4917462 | Apr., 1990 | Lewis et al. | 350/319.
|
| 4947034 | Aug., 1990 | Wickramasinghe et al. | 359/368.
|
| 5004307 | Apr., 1991 | Kino et al. | 359/368.
|
| 5018865 | May., 1991 | Ferrell et al. | 250/227.
|
| 5081350 | Jan., 1992 | Iwasaki et al. | 250/234.
|
| 5105305 | Apr., 1992 | Betzig et al. | 359/368.
|
| 5214282 | May., 1993 | Yamaguchi et al. | 250/307.
|
| 5254854 | Oct., 1993 | Bedzig et al. | 250/234.
|
| 5272330 | Dec., 1993 | Betzig et al. | 250/216.
|
| 5289004 | Feb., 1994 | Okada et al. | 250/306.
|
| 5382789 | Jan., 1995 | Aoshima | 250/216.
|
Other References
H. Giachemi, et al., "Low Temperature Near Field Spectroscopy and
Microscopy", Journal: Ultramicroscopy, vol. 57, pp. 165-168, Jan. 1995.
|
Primary Examiner: Allen; Stephone
Attorney, Agent or Firm: Baker & Botts, L.L.P.
Claims
What is claimed is:
1. A near-field microscope for examining a sample comprising:
an optical fiber probe having a probe tip;
a collection fiber distinct from said optical fiber probe, said collection
fiber being disposed substantially co-axially with said probe tip and a
portion of said sample being scanned;
detection means for detecting light reflected or emitted from said sample;
a first light source coupled to said optical fiber probe and a second light
source coupled to said collection fiber;
wherein said near-field microscope may be selectively operated in either
collection mode or illumination mode.
2. The near-field microscope of claim 1 wherein said optical fiber probe is
disposed through an aperture in a lens, said lens being positioned
co-axially with said probe tip and a portion of said sample being scanned.
3. The near-field microscope of claim 1 wherein said collection fiber is
disposed above said sample and said microscope operates in reflection
mode.
4. The near-field microscope of claim 1 wherein said collection fiber is
disposed below said sample and said microscope operates in transmission
mode.
5. A near- field scanning optical microscope for examining a sample
comprising:
an optical fiber probe having a probe tip;
a light source;
a first lens and a second lens, said first lens being disposed between said
second lens and said staple, and said optical fiber probe and probe tip
being positioned through an aperture in said first lens; and
detection means coupled to said optical fiber probe for detecting light
reflected or emitted from said sample;
wherein said first and said second lenses collectively focus said reflected
or emitted light on a collection fiber.
6. The near-field microscope of claim 5 wherein said collection fiber is
connected to a photodetector for processing said reflected or transmitted
light.
7. A near- field scanning optical microscope for examining a sample
comprising:
an optical fiber probe having a probe tip;
a light source;
at least one lens, said optical fiber probe and probe tip being positioned
through an aperture in said at least one lens;
detection means coupled to said optical fiber probe for detecting light
reflected or emitted from said sample; and
a collection fiber, said at least one lens being disposed between said
collection fiber and said sample;
wherein said collection fiber is distinct from said optical fiber probe.
8. A near field scanning optical microscope for examining a sample,
comprising
an optical fiber having a probe tip;
at least one lens, said optical fiber probe and probe tip being positioned
through an aperture in said at least one lens such that said probe tip is
positioned adjacent a sample to be examined;
an optical fiber disposed opposite said at least one lens from the sample
to be examined;
wherein said optical fiber probe is adapted to emit light through said
probe tip and said optical fiber is adapted to receive light when
operating in an illumination mode, and said optical fiber probe is adapted
to receive light through said probe tip and said optical fiber is adapted
to emit light when operating in a collection mode.
9. The near field microscope of claim 8 wherein said probe tip is
positioned at the focal point of said lens.
10. The near field microscope of claim 9 having a first and a second lens,
wherein said first lens is disposed between said second lens and said
sample and said optical fiber probe and probe tip are positioned through
an aperture in said first lens.
11. The near-field microscope of claim 9 wherein said optical fiber probe
is coupled to a photodetector when operating in said collection mode and
said optical fiber probe is coupled to a light source when operating in
said illumination mode.
12. The near field microscope of claim 9 wherein said optical fiber probe
is a single mode optical fiber.
13. The near-field microscope of claim 9 wherein said optical fiber is
coupled to a photodetector when operating in said collection mode and said
optical fiber is coupled to a light source when operating in said
illumination mode.
14. The near field microscope of claim 9 further comprising means for
detecting the distance between said probe tip and said sample.
15. The near field microscope of claim 14 wherein said means for detecting
the distance between said probe tip and said sample comprises means for
detecting shear force.
16. A near-field microscope for examining a sample, comprising:
an optical fiber probe having a probe tip;
a collection fiber distinct from said optical fiber probe, said collection
fiber being disposed along an optical path with said probe tip and a
portion of said sample being scanned;
detection means for detecting light reflected or emitted from said sample;
a first light source coupled to said optical fiber probe and a second light
source coupled to said collection fiber;
wherein said near-field microscope may be selectively operated in either
collection mode or illumination mode.
Description
FIELD OF THE INVENTION
This invention relates generally to optical microscopy and more
specifically to illumination, collection and shear force detection in a
near-field scanning microscope.
BACKGROUND OF THE INVENTION
Microscopes employing conventional optical imaging systems are limited in
their resolution capabilities. It is known that conventional optical
microscopy techniques can not be used to resolve features significantly
smaller than one-half the wavelength of the light used to illuminate the
sample. As a result, transmission and scanning electron microscopes were
developed in order to provide the ability to examine structures
substantially smaller than the wavelength of visible light. In fact,
technologies such as scanning tunneling microscopy (STM) have allowed the
resolution of structures as small as individual atoms.
Unfortunately, these high resolution techniques suffer from the drawback
that they require that the sample be placed in a vacuum and/or be
subjected to ionizing radiation. This requirement has proved
unsatisfactory for many types of specimens (e.g. biological materials)
since considerable damage to the specimen or modification of the property
to be investigated often results from observation or sample preparation.
Moreover, most of these techniques employ tunnelling electrons or an
electron beam as the signal source within the microscope. Thus, a sample
must be generally electrically conductive in order to be observable.
In addition to these problems, microscopes based upon tunnelling electrons
are unable to fulfill the requirements of researchers who wish to study
the electrical or optical properties of an object. Certain features of a
specimen that are detectable by optical microscopy may nevertheless be
invisible to an electron microscope because the two devices measure
substantially different physical properties. For example, while electron
microscopes are suited quite well for examining surface topology, they are
practically unusable for studying electro-optic and/or optical properties
of specimens such as active semiconductor devices or biological samples.
SEMs (scanning electron microscopes), TEMs (transmission electron
microscopes) and STMs provide primarily structural information. These
types of instruments can not adequately provide information on a
specimen's optical properties such as color, reflectance, fluorescence and
luminescence.
In response to these problems, near-field scanning optical microscopy
(NSOM) has been developed to achieve fine resolution (well below the
one-half wavelength diffraction limitation) without any resultant damage
to the observed sample. NSOM is a relatively recent technology that has
been employed for both the imaging and spectroscopy of materials at
resolutions far below the familiar diffraction limit. NSOMs are capable of
measuring a variety of optical properties associated with a sample. An
NSOM generally consists of an aperture having a diameter that is smaller
than an optical wavelength which is positioned in close proximity to the
surface of a specimen and scanned over the desired portion of the sample.
The light thus exiting the aperture is largely independent of the
wavelength of the incident light.
As the aperture is moved across the sample, an optical response of the
specimen to the near-field is produced, and the resulting photons are
detected by a remote photodetector. Conventional means are then employed
to collect and assemble data such that a scanned image corresponding to
the sample is produced for viewing.
NSOMs generally require some method for determining and maintaining a
particular distance between the probe tip and the sample surface. This is
often referred to as z-axis control. Shear force topographic imaging
(dithering) has emerged as one technique for use in NSOMs. This method is
sensitive, non-destructive, sample independent and provides a wide dynamic
signal range for distances up to 50 nm above the sample. Typically, the
tip is dithered by mounting it on a piezoelectric tube. As a result, prior
art devices employing the dithering technique for z-axis control have
heretofore used off-axis objectives for the collection and illumination of
the tip region.
The imaging capabilities of super-resolution devices such as the NSOM are
desirable in a broad range of disciplines ranging from semiconductor
devices and materials to biological systems and beyond. For example,
coupled with sensitive spectroscopic probes, NSOM can provide an
unprecedented level of diagnostic capabilities to investigate and
understand the optical and electro-optic properties of active
semiconductor devices on a better than 30 nm length scale (.lambda./20) in
the visible light region. Additionally, optical modes in optoelectronic
devices can be mapped, local doping profiles can be determined and
fabrication process and lattice mismatch induced strains can be
ascertained.
NSOM devices further provide the ability to map photoluminescence (PL) and
electro-luminescence (EL) emission at subwavelength resolution. PL can
determine defect type and density relative to band edge emission by
examining intensity ratios. PL wavelength shifts in band edge emission are
indicative of local strain fields. EL is used to understand the behavior
of active opto-electronic devices. Additionally, using NSOM in
illumination-transmission mode (discussed below), single molecules can be
imaged using near-field fluorescence microscopy. Site specific near-field
fluorescence microscopy can provide novel information on biological
systems.
Often, the aperture in NSOM devices is provided in the form of a tapered
single mode optical fiber with a typical aperture of 20-200 nm. The fiber
tip is placed within the near optical field of the sample. Because both
the tip to sample separation and the tip aperture are a small fraction of
the visible light wavelength, the resulting spatial resolution is not
limited by the usual far field Rayleigh criteria of .lambda./2. In NSOM,
the electric and magnetic fields at the sample are effectively confined to
the tip diameter, and therefore can yield resolutions as high as
.lambda./40, or about 15 nm for visible wavelengths.
NSOM devices operate primarily in one of two distinct modes. In the first
possible mode, illumination mode, the excitation light is directed down
the tapered optical fiber tip, and the transmitted, reflected or emitted
light is collected by far-field optics. In the second mode, collection
mode, the sample is excited by far-field optics, and the transmitted,
reflected or emitted signal is collected in the near-field by the fiber
tip. Collection mode operation is typically employed when examining
semiconductor and opto-electronic systems. This is because when examining
these types of samples, excitons diffuse from the excitation point prior
to recombination. It is thus beneficial to use the near field resolution
to collect light rather than to excite the sample. In contrast, biological
systems are better suited to illumination mode, since the fluorescence
emanates from localized centers.
NSOM operation can be further characterized according to directional
relationship by which light is collected. In a first procedure, incident
light (produced either in illumination mode or collection mode) is
transmitted through the sample and collected below the stage. The
collected light is directed towards a photodetector device and the image
is reconstructed. This method is referred to as transmission mode and is
commonly employed with transparent or semi-opaque samples such as
biological specimens. Alternatively, an NSOM may operate according to a
reflection mode whereby light is reflected or emitted from the sample
surface and collected either in the near field (collection mode) or in the
far field (illumination mode) somewhere above the sample surface.
Opaque samples require the use of reflection mode. One reflection mode
technique which has been used with some degree of success has been
suggested by R. D. Grober et al. ("Design and Implementation of a Low
Temperature Near-Field Scanning Optical Microscope", Rev. Sci. Instr.,
March 1994). Grober calls for placing an optical fiber tip at the focal
point of a reflecting objective. The resulting apparatus provides
dispersionless optics using a microscope objective having a small primary
convex mirror and a large secondary concave mirror. The mirrors are
mounted on plates capable of vertical motion for focusing and collimating
the luminescence. Grober has reported that he has been able to achieve a
numerical aperture (NA) value of 0.4.
The Grober device, however, suffers from a number of disadvantages.
Firstly, the mirroring apparatus requires a motion control separate from
the typical x, y and z dimension motion controls used to move the sample
platform and/or the fiber probe. As a result, the device is more costly
than a non-reflecting objective based counterpart device. Moreover, the
inclusion of the mirrors and their associated motion controls increases
the physical dimensions of the optical system contained within the NSOM
device. An assembly that is not compact in size is often impossible to
incorporate within an existing conventional microscope. In addition,
specialized environments such as vacuums and cryogenic chambers often can
not accommodate a bulky assembly such as that required with the Grober
design.
As described above, opaque specimens require the use of a reflection mode
NSOM device. Heretofore, the operation of NSOMs in reflection mode has
occurred almost exclusively through the use of off-axis collection
objectives. One known exception is a device described in U.S. Pat. No.
4,725,727 issued to Harder et al. This patent appears to describe a
generally co-axial scheme using a waveguide formed from quartz crystal. A
tip is formed and two opaque layers are deposited on the tip such that,
for example, an inner transparent layer may be used to illuminate the
sample, with the reflected light being captured by the outer layer and
delivered to photodetectors. This scheme, however, requires multiple,
complicated coatings on the tip and is thus difficult to implement in
practice. In addition, this scheme can not perform any imaging of the
sample region about the tip to, for example, direct the tip over the
sample region to be studied.
FIG. 1 illustrates a prior art near-field optical microscope operating in
the reflection mode. An example of an NSOM using such an off-axis
objective in a reflective geometry is the Aurora TMX2000 model built by
the Topometrix Corporation located in Santa Clara, Calif. For purposes of
illustration, however, a distinct prior art reflective mode NSOM is shown
in FIG. 1 and is discussed herein.
As is apparent to one of ordinary skill in the art, the prior art NSOM of
FIG. 1 includes a probe 10 terminating in a probe tip 70. A stage 20 is
further provided for supporting sample 30. The probe may be displaced
relative to the sample in the x, y and z dimensions by means of
piezoelectric actuators 40. A light source 60 is employed to illuminate
probe tip 70 and a photodetector 80 is provided for the detection of a
change in amplitude or phase of the vibrating (dithering) tip.
Off-axis, side mounted objective lens 120 is used to image the tip region,
provide illumination, or collect the reflected or emitted light from the
sample as a result of illumination through probe 10 and tip 70 by way of
source 60. In a typical off-axis scheme, the objective lens 120 may be
mounted at a distance of approximately 10 mm from the tip 70.
Such an off-axis scheme suffers from a small collection efficiency and
reduced resolution. This is because the use of an off-axis objective
requires independent x, y, z controls at the off-axis objective. Because
of this design, a relatively large distance between the sample 30 and
objective 120 is needed. This, in turn, results in a smaller numerical
aperture for collecting the reflected or emitted light resulting in
decreased performance. Further, as a result of this reduced collection
efficiency, scan speed is generally reduced in order to achieve a
satisfactory resolution. Alternatively, if scan speed is maintained, the
resolution will suffer.
In contrast, if it is possible to place the collecting device (i.e. the
collection objective) in co-axial alignment with the probe tip, the
collection efficiency and thus the resolution and brightness will improve
dramatically. This is because a higher numerical aperture can be achieved
by co-axial detection as a result of a reduction in distance between the
probe tip and the collection objective. For example, this distance may be
reduced by a factor of approximately five to 2 mm by using a co-axial
design.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an improved
method and apparatus for performing near-field optical microscopy.
It is a further object of this invention to provide a near-field optical
microscope having a high degree of collection efficiency allowing
detection of low-intensity visible light while simultaneously allowing
piezoelectric dithering of the probe tip for shear force imaging.
It is a still further object of the present invention to provide a co-axial
collection scheme in a near-field optical microscope.
It is an even further object of the present invention to provide such a
co-axial collection scheme in a compact, simple and easily manufactured
form.
It is a yet further object of this invention to provide a method of
detecting the light emanating from, illuminating the region about, or
imaging the region about a tip in a near-field optical microscope.
It is a still further object of this invention to provide a near-field
optical microscope having a relatively low cost optical system.
In accordance with the present invention, a near-field optical microscope
is provided having a tip or tapered optical fiber light guide mounted
directly in a hole drilled in the center of a microlens which itself is
mounted in a piezoelectric tube for dithering. The tip of the tapered
fiber is positioned at the front focal point of the lens. Light emanating
from the region just about the tip is thus collected and collimated by the
lens. A second lens, further down the optical path next focuses the beam
into another fiber for transmission to a photon detection and/or
spectroscopic apparatus. This second fiber can alternatively be replaced
by one or more image processing lenses or a coherent fiber bundle with the
light being transmitted to a position sensitive detector to provide a
direct image of the tip region. The microscope also includes a shear force
dithering apparatus for control of fiber tip to sample distance.
The present invention provides an efficient and compact design and method
for co-axial detection and illumination of a sample region about the tip
while at the same time affording the ability to dither a pulled fiber tip
for shear force imaging. The device may be incorporated within an existing
conventional microscope. Additionally, the device may be used in
connection with specialized environments such as vacuum and cryogenic
chambers.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and additional objects, features, and advantages of the
present invention will become apparent based upon the following detailed
description to be taken in conjunction with the accompanying drawings in
which:
FIG. 1 is a schematic representation of a prior art near-field optical
microscope operating in the reflecting mode;
FIG. 2 is a schematic drawing of the NSOM according to this invention;
FIG. 3 is a schematic drawing of a portion of the NSOM illustrated in FIG.
2 according to one embodiment of this invention;
DETAILED DESCRIPTION
FIG. 2 illustrates an NSOM according to one embodiment of this invention.
The NSOM preferably includes vibration isolation table 140 and device
probes 150. Device probes 150 may be employed to, for example, provide
electrical contact to opto-electronic devices or to apply potentials to
biological samples. Also included is a sample motion piezoelectric 170 for
moving the sample in x, y and z directions with respect to probe tip 110.
Piezoelectric tube 130 may provide motion of the probe tip 110 for fine
control optimization of a shear force signal. The light from light source
165 is coupled to probe 135 through switch 184 and is selectively
transmitted to probe tip 110. Photodetector 180 is included for detecting
light collected by probe tip 110. Switch 184 is preferably a mirror.
Photodetector 180 is preferably a photomultiplier (PMT) but may be a
spectrometer for wavelength dispersion and photon detection or any
photodetector capable of detecting photon impingement. Collection fiber
148 is connected to light source 190 and photodetector 185 through switch
183.
The probe 135 is manufactured by heating a single-mode optical fiber to
soften it, and drawing the softened fiber to form a tapered fiber. After
drawing, at least a portion of the tapered portion, not including the
aperture 350, is preferably coated with a reflecting opaque material such
as a metal.
A long-wavelength laser diode 172 and InGaAs quadrant photodetector 173,
with associated steering optics and lock-in amplifier are further provided
for shear force excitation and detection. Alternatively, photodetector 173
can be any semiconductor photodetector paired with a suitable wavelength
laser diode. Dithering piezoelectric 132 is employed to vibrate probe tip
110 in connection with the shear force detection mechanism as discussed
below. It will be understood by one of ordinary skill in the art that such
shear force detection is useful in NSOM in order to maintain the probe tip
110 at a constant, or approximately constant distance from the sample
surface.
A lens objective consisting of a primary lens 105 and a secondary lens 138
are fitted within dithering piezoelectric 132. This lens objective is
described in further detail below in conjunction with FIG. 3.
FIG. 3 illustrates a novel optical system according to one embodiment of
the present invention. The probe 135, which is preferentially provided in
the form of a single-mode optical fiber, is placed within dithering
piezoelectric 132 and through a hole formed essentially in the center of
primary lens 105. The optical fiber, as described above, is tapered and
positioned with tip 110 at the front focal point of primary lens 105.
Alternatively, graded index lenses (GRIN) could replace lenses 105 and
138.
In a preferred embodiment of this invention, scanning tube 130 comprises an
approximately 0.5 inch long, 0.25 inch diameter, four quadrant
piezoelectric tube which is used to provide fine control of probe tip
position for optimizing the shear force signal. At the base of scanning
tube 130, a macor piece 330 with integral secondary collection microlens
138 and an approximately 200 .mu.m collection fiber 148 is used to mount
the approximately 0.125 inch diameter dithering piezoelectric tube 132.
The probe tip 110 is preferably installed through an approximately 0.013
inch hole formed in primary lens 105 affixed to the end of dithering
piezoelectric 132. Probe tip 110 is drawn to provide an aperture 350 of
approximately 50 nm and is preferably coated with a metal layer
approximately 100 nm thick. This metal may be, for example, aluminum. The
image of collection fiber 148 is focussed at the probe tip 110, preferably
0.080 inches from primary lens 105.
Reflection geometry illumination mode operation is obtained by illuminating
the sample 125 through probe tip 110 and collecting the reflected and
emitted light back up through primary lens 105 and secondary lens 138 into
collection fiber 148. Collection fiber 148 may be selectively connected to
photodetector 185 or light source 190 through switch 183. Light source 190
and light source 165 may be the same component or they may be provided
separately. Collection mode operation occurs in reverse. In this case,
collection fiber 148 is used to illuminate a broad area of the sample,
with the probe tip 110 collecting reflected or emitted light in the
near-field at the sample surface.
The microscope of this invention may alternatively be operated in
transmission mode when a transparent or semi-opaque specimen is examined.
For purposes of illustration, an illumination-transmission geometry is
discussed although it will be understood by one of ordinary skill in the
art that this invention could equally be implemented in a
collection-transmission mode. In the illumination-transmission mode, light
is emitted by light source 165 through probe 135 and probe tip 110. Light
transmitted or emitted from the sample 125 is captured in the far field
through GRIN lens 390 and into optical fiber 395. Optical fiber 395 is
connected to a photodetector device as is known in the art and processing
electronics (not shown) can process the image for viewing. Alternatively,
a coherent optical fiber bundle may be used in place of optical fiber 395,
or an objective lens may be used in place of optical fiber 395 and lens
390, in order to provide additional far field imaging capabilities.
In typical operation under illumination mode with reflection geometry,
piezoelectric 170 scans the sample 125. Light collected by lens 105 and
passing through lens 138 to collection fiber 148 creates the near-field
optical image thus allowing probe to sample distance to be controlled by a
feedback loop. Laser diode 172 and photodetector 173 of FIG. 2 are used to
obtain a simultaneous shear force signal. In this way an image at constant
tip to sample separation is generated. It is critical to the NSOM
technique that optical data be collected in the near-field while at the
same time performing the shear force detection described below.
Electronics and processors 145 used to control the scan and the shear
force feedback loop are readily available from various scanning probe
microscopy suppliers such as Topometrix and Park Scientific.
Course vertical approach is provided by a differential screw which drives a
differential spring. The spring, in turn moves a piston ground to close
tolerance with a matching cylinder (not shown). In typical operation, a
tip to sample separation at room temperature of 80.mu.m (a single turn on
the differential screw) changes less than 10% at low temperature. The
sample 125 is preferably mounted on an approximately 2.0 inch long sample
motion piezoelectric 170 which can also be used for coarse positioning of
the sample 125 using the inertial motion or a slip stick technique.
The NSOM according to this invention provides a numerical aperture of
approximately 0.55. This compares favorably with off-axis objective based
devices such as that manufactured by Topometrix which has a numerical
aperture of 0.35. Taking into account the shadowing of the central region
of the light cone by the probe tip 110 which results from the use of a
coaxial scheme, it is still possible to achieve a significant improvement
in efficiency. Moreover, the optical system of the present invention
results in a significantly smaller design. One typical off-axis technique
requires 3.times.3.times.1 cubic inches to contain the detection
components located off-axis. In comparison, the detector components will
fit in a volume of 0.25.times.0.25.times.0.3 cubic inches through the use
of the co-axial scheme of this invention.
Since the optical response is generally not representative of surface
topography, an independent measure is necessary to maintain a fixed
proximity (about 5 nm) between probe tip 110 and sample 125. This is
accomplished by a shear-force detection mechanism whereby the probe tip
110 is dithered (vibrated) by dithering piezoelectric tube 132 at the
mechanical resonant frequency of probe tip 110. This is typically 30-130
kHz. An optical beam originating at laser diode 172 and detected by InGaAs
quadrant photodetector 173 operates to monitor the amplitude and/or phase
of the probe tip vibration. Any tip-surface interaction quenches the
resonance and shifts the resonant frequency, providing a height
measurement with sub-nanometer resolution. In another embodiment, a 100
.mu.m fiber with attached GRIN lens focusses a 1.310 .mu.m laser from
laser diode 172 onto probe tip 110. A fiber and GRIN lens pair is mounted
directly opposite, collecting the shear force signal in transmission,
while another pair is preferably mounted at 90.degree. to collect in
reflection. This method is preferably practices for remote or cryogenic
applications.
In an additional embodiment of the invention, the lens and probe
combination may be used as a top mounting scheme providing only
illumination optics. This affords a simple way to illuminate the region
just about the probe tip, without the use of off-axis illumination
sources. This scheme could be used, for example, in conjunction with a
conventional optical microscope.
While the invention has been particularly described with reference to
particular embodiments thereof, it will be understood by those skilled in
the art that various other changes in detail may be made therein without
departing from the spirit, scope or teachings of this invention.
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